Cannot Upgrade Pro Plugin

My ex-developer bought the Relevanssi Premium plugin for my client’s website, which he built in 2011. I’m not sure whether Relevanssi was installed then or later though. The plugin has stopped updating at v2.16.5. I can’t login at Relevanssi to check the license because it was bought under his account, not mine. Why has it stopped updating, and how can I update to the latest version?